This invention relates to monolithic crystal filter structures and more particularly to monolithic crystal filter structures exhibiting attenuation peaks at the edges of the filter passband.
The term "monolithic crystal filter" is used herein to define the basic filter structure disclosed in U.S. Pat. No. 3,564,463, issued on Feb. 16, 1971 to W. B. Beaver and R. A. Sykes. As disclosed in the aforesaid patent, such a monolithic crystal filter comprises a plurality of resonators which share a common piezoelectric wafer. More particularly, a monolithic crystal filter is characterized by two distinguishing features, namely, mass loading and acoustic coupling. The term mass loading refers to a particular electrode mass which is determined by the nature of the piezoelectric body and its thickness, and by the size and density of the electrodes which make up each of the resonators. Acoustic coupling, on the other hand, refers to the existence of an energy channel in the piezoelectric body which effects the transmission of acoustic energy between the input and output electrodes. By virtue of a specific combination of mass loading and acoustic coupling, the image impedance of the monolithic crystal filter structure as a whole conforms to a specifically defined pattern. Additionally, the structure, as a whole, has an equivalent circuit in the form of a lattice network with resonant and antiresonant frequencies characterized by a specifically defined relation.
In order to ensure the sharpest possible cutoff action and corresponding high degree of selectivity, the attenuation characteristic of any effective band filter, including a monolithic crystal filter, should be marked by steep skirts of attenuation near the filter passband. In the case of a monolithic crystal filter, one particular technique for achieving such steep attenuation skirts has been to modify the conventional monolithic filter structure such that the structure is made to exhibit attenuation poles close to the filter passband. Unfortunately, however, the modifications used to date in realizing such attenuation poles have involved the adding of other signal paths and/or resonators to the basic filter structure, thereby tending to increase its complexity.
It is therefore a broad object of the present invention to improve the attenuation characteristics of a conventional monolithic crystal filter by establishing controllable attenuation poles relatively close to the filter passband in a manner which does not overly increase the complexity of the filter structure.